In telecommunication, a waveguide is a material medium that confines and guides a propagating electromagnetic or optical wave. In the microwave regime, a waveguide normally consists of a hollow metallic conductor, usually rectangular, elliptical, or circular in cross section. This type of waveguide may, under certain conditions, contain a solid or gaseous dielectric material. In the optical regime, a waveguide used as a long transmission line consists of a solid dielectric filament (optical fiber), usually circular in cross section. In integrated optical circuits an optical waveguide may consist of a thin film that is optically transparent for the wavelengths of interest. Waveguide propagation modes depend on the operating wavelength and polarization and the shape and size of the guide. The present invention is concerned with optical waveguides.
Gratings are optical devices used to achieve wavelength-dependent characteristics by means of optical interference effects. These wavelength-dependent optical characteristics can, for instance, serve to reflect light of a specific wavelength while transmitting light at all other wavelengths. Gratings are usually implemented by modulating (varying) the effective index of refraction of a wave-guiding structure. The variation of refractive index along the length of the grating is often referred to as the “index profile” of the grating. These changes in index of refraction cause incident light to be reflected.
Gratings may be “written” into the optical waveguide in a variety of different ways, depending primarily on the material used. Fiber or glass guides, for example, often make use of photo refractiveness, a property of specially prepared glasses that allows their refractive index to be varied by exposing them to high intensity light (typically in the ultraviolet), termed photo inscription. Semiconductor gratings, on the other hand, are usually implemented as surface-relief gratings by etching a grating pattern into the surface of the semiconductor guide (which may then be buried following subsequent deposition). Etching the surface of the waveguide does not affect the true refractive index of the optical medium as photo inscription does, but rather varies the guide's effective index. Nevertheless, this difference does not affect the operation of the grating.
A simple and common grating device is a Bragg Grating, which consists of a periodic variation in refractive index and acts as a reflector for a single wavelength of light related to the periodicity (known as pitch) of the index pattern. It is frequently used in both semiconductor systems and fiber-optic systems, where it is known as a Fiber Bragg Grating. The Bragg Grating can actually reflect at several wavelengths, corresponding to overtones of its fundamental pitch. However, higher order wavelengths reflected from a Bragg grating tend to be at quite different spectral regions than the fundamental, so Bragg Gratings are not generally useful as a multi-wavelength reflector. Moreover, these higher-order wavelengths cannot be tuned independently of one another.
There are several multi-wavelength grating technologies: analog superimposed gratings, Sampled Gratings (SG), Super-Structure Gratings (SSG) and Binary Supergratings (BSG). The binary supergrating is also known as a binary superimposed grating, for historical reasons. BSG development was originally motivated by a desire to emulate the superposition of multiple conventional Bragg gratings, hence the term “binary superimposed grating”. Since then, synthesis techniques have evolved to allow the emulation of arbitrary diffraction characteristics, a flexibility better captured by the term “binary supergrating”.
Diffraction gratings in combination with guided light inside waveguides are being used to create novel wavelength division multiplexing (WDM), or to provide wavelength-specific feedback for tunable or multi-wavelength semiconductor lasers. Wavelength Division Multiplexing (WDM) is a technology where many communication channels are encoded into a single optical cable by utilizing different wavelengths of light. Gratings are often used to separate or process these channels. Older grating technologies can process one wavelength at a time, forcing devices intended to process multiple wavelengths to employ a cascade of single-wavelength gratings. This is not an attractive solution because, on top of the additional losses that each grating creates, even a single grating occupies a considerable amount of space by today's standards of integration. It is thus desired to have a single device capable of processing several wavelengths (or ranges of wavelengths) in a space-efficient manner.
Early WDM systems were expensive and complicated to run. However, recent standardization and better understanding of the dynamics of WDM systems have made WDM much cheaper to deploy. The market has segmented into two parts, “dense” and “coarse” WDM. Dense WDM (DWDM) is generally held to be WDM with more than 8 active wavelengths per fibre, with systems with fewer active wavelengths being classed as coarse WDM (CWDM). DWDM in carrier networks promises substantial increases in the capacity of carrier backbones. To avoid the need for multiple lasers, each tuned to a different wavelength, carriers have used tunable lasers such as those noted above, so-called Distributed Feedback lasers or DFBs. First generation tunable lasers could be configured to two or possibly four different wavelengths, whereas newer generation tunable lasers are capable of being tuned over a much wider range of wavelengths and switchable between them at speeds fast enough to route packet-based traffic as optical routers. Tunable gratings have developed alongside tunable lasers, and generally have a fixed pattern whose index is altered by current injection, heating, or the like. However, while those tunable gratings may produce a shift in spectral characteristics (e.g., a phase shift), the spectral response is constrained to a fixed overall shape.
Some research has gone into overlaying single frequency gratings over one another, in order to minimize the physical space taken up by the multiple gratings required for wavelength-specific feedback to tunable lasers. See, for example, Design of Widely Tunable Semiconductor Lasers and the Concept of Binary Superimposed Gratings (BSGS), by Ivan A. Avrutsky et al, IEEE JOURNAL OF QUANTUM ELECTRONICS, vol 34, no. 4, April 1998. The above Avrutsky article describes the binary gratings as an array of gratings superimposed analogly, and then subjected to binary digitization according to the desired wavelength. The resulting binary supergrating is a single-depth grating of a constant period. Notwithstanding the advantages offered by the Avrutsky superimposed gratings, there remains a need for enough physical space to dispose multiple iterations of the gratings where more than two channels are present. The addition of more channels will require additional physical space. Further, the very patterning of physical gratings by photolithography or etching is an intricate and expensive manufacturing process requiring a relatively high level of quality control to ensure the grating separates optical transmissions according to the desired wavelength.
What is needed in the art is an optical grating whose interference with an optical wave propagating in a waveguide is selectable. Such a grating would have greatly increased utility if it were small enough to be adapted to semiconductor waveguides.